In a conventional power-supply apparatus, as illustrated in FIG. 8, a circuit for protecting a switching element outputs from an output terminal OUT an output voltage Vout, from an input voltage Vin input into an input terminal IN. A circuit is generally used that compares with a reference voltage Vs a voltage drop due to a fixed resistor Ra connected in series with a PMOS transistor Ma constituting a switching element. When the voltage drop as described above exceeds the reference voltage Vs, the circuit controls a gate voltage of the switching element Ma such that the impedance of the switching element Ma is increased so as to limit a current output from the output terminal OUT.
Moreover, FIG. 9 is an example circuit of a power-supply apparatus having combined a switching element Ma with a constant-voltage circuit. In the case of FIG. 9, the on-state resistance of the switching element Ma is set smaller than that of a voltage-controlling transistor Mb constituting the constant-voltage circuit. Thus, turning on the switching element Ma when a voltage Vin of an input terminal IN is at or below a rating output voltage of said constant-voltage circuit makes it possible to reduce a voltage difference between the input voltage Vin and an output voltage Vout.
Now, when the input voltage Vin reaches the rating output voltage of said constant-voltage circuit so as to make it possible for the constant-voltage circuit to output the rating output voltage, a control signal input to a gate of the switching element Ma turns off the switching element Ma so that the output voltage Vout is clamped at the rating output voltage of said constant-voltage circuit.
Moreover, in case the input voltage Vin reaches below the rating output voltage of said constant-voltage circuit so that the switching element Ma is turned on, when such accidents as a load short-circuiting, etc., occur, as the on-state resistance of the switching element Ma is small, an excess current flows from the input terminal IN through the switching element Ma so as to produce a defect in the switching element Ma. In order to protect the switching element Ma from such excess current, a current-controlling circuit as illustrated in FIG. 8 is added that connects a fixed resistor Ra in series with a switching element Ma. Furthermore, there is an excess-current protection system for making it possible to prevent destruction of semiconductor switches such as a MOSFET, etc., without using an IPS with a built-in fuse and an excess-current protection circuit (refer to Patent Document 1, for example).
Patent Document 1
Japanese Patent Laid-Open Publication 09-046200
However, in the conventional circuits as illustrated in FIGS. 8 and 9, there is a problem such that adding to a voltage drop caused by the switching element Ma on its own, the voltage drop due to the fixed resistor for current detection causes a fall in the output voltage Vout to become large. More specifically, in FIG. 9, when operating in a state such that the input voltage Vin is smaller than the rating output voltage of the constant voltage circuit, setting the voltage difference between the input terminal IN and the output terminal OUT as small as possible is desired. However, no matter how small the on-state resistance of the switching element Ma is set to be, due to the fixed resistor Ra, there is a limit to reducing the impedance between the input terminal IN and the output terminal OUT.